Hi the dc-dc converter or dropper is behind the passenger dash 
You gain access  from the bonnet area on the right remove the black plastic cover 10mm bolts I think 
I had this fail due to a poorly replaced windscreen leaking water into the converter killing it blowing fuse etc on dash left side I think..
I purchased an aftermarket one and that's what mine runs on now
Hope this helps. Craig...
                
 craig
                
 January 2013